Present "state of the art" has created the altogether unsettling situation wherein a critically ill patient is admitted to the hospital with no infarcted brain tissue and yet, after a period of extremely intense and expensive critical care, is discharged with new hospital-acquired dead brain tissue. Our long range goal and overall objective is to prevent the development of in- hospital brain tissue death through development of improved bedside rCBF monitoring techniques that will enable implementation of individualized therapy to prevent such adverse outcomes with their attendant long term disabilities. Early studies suggest near infrared spectroscopy (NIRS) techniques that involve diffuse correlation spectroscopic (DCS) continuous CBF measurement have promise to be developed into valuable bedside CBF monitors. Our specific aims are to test the hypotheses in NeuroICU patients with subarachnoid hemorrhage (SAH) that: 1. Changes in DCS rCBF accurately reflect acute changes in XeCTCBF a. XeCTCBF and DCS will concurrently be determined before and after blood pressure manipulation followed by analysis for degree of correlation of the fractional changes in the two measures. 2. DCS rCBF accurately reflects vasospasm-induced changes in XeCTCBF a. Continuous DCS will be employed continuously and serially compared with XeCTCBF followed by analysis for degree of correlation of the fractional changes in the two measures Stable xenon computed tomographic rCBF (XeCTCBF) will provide a detailed and quantitative CBF map throughout the brain...the Gold standard. DCS CBF will be determined over the six major cerebral artery distributions. Fractional changes in matched regional XeCTCBF's and DCS CBF will be compared through correlation methods and Bland-Altman analysis also will be used to test agreement between XeCTCBF and DCS. The DCS CBF technique, if valid, will likely eventually become an important tool for clinicians in their efforts to prevent new brain tissue death in patients admitted with ischemic stroke, intracerebral hemorrhage, subdural hemorrhage, subarachnoid hemorrhage, and traumatic brain injury. These diseases constitute the leading cause of disability in the United States. PUBLIC HEALTH RELEVANCE: Narrative Management of neurologically critically ill patients is hindered by inability to easily, repeatedly, and noninvasively, at the bedside, determine regional cerebral blood flow (rCBF). The proposed research will validate in healthy volunteers the Diffuse Correlation Spectroscopy Brain Blood Flow method which may provide the ability to do continuous bedside rCBF monitoring in critically ill patients. This technique, if valid, will eventually help prevent new neural death in patients admitted with ischemic stroke, intracerebral hemorrhage, subarachnoid hemorrhage, and traumatic brain injury by enabling individualized titration of ICU therapies against an important neurophysiologic parameter. These diseases constitute the leading cause of disability in the United States..